Load balancer

ABSTRACT

A pneumatic load balancer having a hoist cylinder adapted for attachment to an overhead support such as a hook, rail, trolley or the like having a piston assembly disposed in the cylinder, a fluid chamber and a lifting cable which is wound around a dual set of pulleys with one end anchored with respect to the cylinder and the other end arranged for attachment to a load. One set of pulleys is fixed against motion near the center of the cylinder and the other set is carried by the piston assembly to reel in or to unwind the cable from the cylinder upon movement of the piston assembly by means of fluid pressure. A pneumatic control system is provided which comprises a monostat in the form a pilot pressure operated fluid pressure regulator having a built-in relief valve and which is fitted to the cylinder inlet and which can be operated either through pilot or mechanical connections to adjust the pressure of fluid available at the inlet of the cylinder in correspondence with the load carried by the cable. Valve means are also provided which are normally open to communicate the fluid chamber with atmosphere and which closes upon an increased pressure in the chamber to thereby dampen upward movement of the cable.

United States Patent McKendrick 1 June 13, 197 2 LOAD BALANCER [57] ABSTRACT [72] Invent Lorne J, M K d i k, 5 1 3] s rf d A pneumatic load balancer having a hoist cylinder adapted for Drive, Milford, Mich. 48042 Primary Examiner-Harvey C. Hornsby Assistant ExaminerMerle F. Maffei Attorney-Hanke, Gifford & Patalidis attachment to an overhead support such as a hook, rail, trolley or the like having a piston assembly disposed in the cylinder, a fluid chamber and a lifting cable which is wound around a dual set of pulleys with one end anchored with respect to the cylinder and the other end arranged for attachment to a load. One set of pulleys is fixed against motion near the center of the cylinder and the other set is carried by the piston assembly to reel in or to unwind the cable from the cylinder upon movement of the piston assembly by means of fluid pressure. A pneumatic control system is provided which comprises a monostat in the form a pilot pressure operated fluid pressure regulator having a built-in relief valve and which is fitted to the cylinder inlet and which can be operated either through pilot or mechanical connections to adjust the pressure of fluid available at the inlet of the cylinder in correspondence with the load carried by the cable. Valve means are also provided which are normally open to communicate the fluid chamber with atmosphere and which closes upon an increased pressure in the chamber to thereby dampen upward movement of the cable.

6 Claims, 3 Drawing Figures PATENTEDJUN13 I972 3,669,411

sum 20F 2 Fluid source INVENTOR LQRNE J. MGKENDRICK LOAD BALANCER BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to a load balancer and more specifically to a pneumatically operated hoist load balancing device.

II. Description of the Prior Art Hoist type load balancers are in common use for supporting heavy loads such as workpieces or tools which have to be positioned relative to the work operation and which can not be conveniently manipulated by hand for any extended period of time.

Pneumatically controlled overhead hoist type load balancers are also known which comprise a cylinder having a piston movable therein, a cable drum operably connected to the piston and a cable extending out of the cylinder for attachment to a load. One end of the cylinder and the piston define a pressure chamber which is adapted to be filled with air or other fluid under pressure to move the piston and thereby operate the cable drum=to either unreel or windup the cable. Prior devices have also included pressure regulators incorporated in the fluid line of load balancing units to provide different pressures acting on the piston in the cylinder chamber to accommodate loads of different weights. The present invention provides an improvement over these known, pneumatically controlled hoist load balancing devices by providing a simplified hoist mechanism in combination with a pilot pressure regulated, adjustable fluid pressure regulator.

SUMMARY OF THE INVENTION The preferred embodiment of the present invention comprises a pneumatically controlled load balancer having a cylinder adapted for attachment to an overhead support such as a hook, rail,'trolley or the like, and in which is disposed a pressure responsive piston assembly forming a fluid chamber between one end of the piston and one end wall of the cylinder. A load supporting cable is Wound around two sets of pulleys with one end of the cable anchored within the cylinder and the other end extending from the cylinder to support a load.

One set of pulleys is fixed against axial motion near the center of the cylinder and the other set of pulleys is attached to and carried bythe piston assembly to be moved with the piston assembly toward and away from the fixed pulleys and to thereby reel-in or unwind the cable from the cylinder. The introduction of fluid under pressure into the hoist cylinder chamber forces the piston assembly away from the center of the cylinder so that the pulleys supported at the opposite end of the piston assembly are moved away from the other set of pulleys fixed near the center of the hoist cylinder to raise the load supported at the other end of the cable. When pressure on the fluid is relieved from the hoist cylinder chamber the piston assembly will be moved by the weight on the cable towards the center section of the hoist cylinder to move the set of pulleys carried at the other end of the piston assembly toward the set of pulleys fixed near the center of the hoist cylinder and to thereby unwind the cable.

The improved pneumatic control system comprises essentially a monostat in the form of a pilot pressure operated fluid pressure regulator having a built-in relief valve which is fitted to the inlet of the balancing hoist cylinder chamber and which can either be remotely pilot operated or directly manually operated. Upon attaching a load to the free end of the balancing hoist cable, the selective adjustment of the pilot pressure operated fluid pressure regulator or monostat acts to regulate the fluid pressure so that the balancer can be used to manipulate loads of different weights. Thus, for any given load the pressure is selected to produce a balancing of the load so that the load can be manipulated, including raising and lowering, by hand.

Once the proper adjustment has been selected, the balancer will then act automatically. Pulling down on the load will cause the monostat inlet pressure controlling section to close and the relief section will detect the increase in fluid pressure within the hoist cylinder chamber due to the compression of the trapped fluid therein. This increased pressure, causes the relief section of the monostat to open, thus bleeding off the excess pressure to maintain a continued balancing'of the load. The decrease in pressure in the chamber which will be caused as the load is relieved by upward manipulation of the load is sensed by the monostat and opens the inlet to pressure. This control arrangement then provides a complete balancing effect over the full cable travel of the balancer.

Thus the present improved construction of a pressure controlled load balancing system provides a constant and effective load balance regardless of the position of the load throughout the travel of the load balancer support cable. The present improved system is readily adjustable upon a change in load value by means of a simple adjustment. Because of the ability to adjust the pressure over a wide range a single balancer can handle loads of various weights so that a single balancer can be used for many different jobs.

Other objects, advantages and novel-features will become apparent by reference to the following description in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS A preferred embodiment of the improved hoist type load balancer and fluid pressure control is illustrated in the accompanying drawings in which:

FIG. 1 is a partially perspective and partially schematic illustration of one preferred embodiment of the present novel load balancing hoist and improved fluid pressure system and control therefore;

FIG. 2 is a cross section through a load balancing hoist device according to the present invention and schematically illustrating a further embodiment of a fluid pressure control system; and

FIG. 3 is a view partially in cross section and partially schematic illustrating the monostat or fluid pressure regulator embodied in either arrangement of FIG. 1 or FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, a pneumatically operated load balancing hoist device generally indicated at 10, is shown as being transversely movably suspended from an overhead rail or beam 12 by means of trolleys 14. A cable 16 extends from the hoist device 10 and is adapted to support a load 18 by means of a load attaching hook 20 or the like. The load 18 may be a heavy workpiece intended to be positioned in a machine fixture on an assembly line or it may be a heavy tool or the like to be handled by an operator. Obviously, the load will be of various weights and in instances, would be far too heavy to be handled without the aid of the hoist device 10.

With particular reference to FIG. 2 a preferred construction of a load balancing hoist device 10 comprises a cylinder housing 22 having one end closed by a front end cap 24 and the other end by a rear end cap 26 to thereby define a cylindrical chamber 28. The chamber 28 is adapted to reciprocably support a dual-piston assembly 30 comprised of a pressure piston 32 and a support piston 34 connected to each other by means of tie rods 36 so as to be movable as a unit. The rear support piston 34 is adapted to support a plurality of pulleys 38 rotatable around a shaft 40. The pulleys 38 are adapted to retain the inner end of the cable 16 which also extends over a second set of pulleys 42 which are rotatably secured within the chamber 28 on a shaft 44 and which are retained stationary relative to the pulleys 38. The end of the cable 16 is anchored within the chamber 28 at a point 46 which is located substantially along a line tangent to the pitch diameters of both sets of pulleys 38 and 42.

Thus, longitudinal movement of the pulleys 38 away from the fixed pulleys 42 causes the cable 16 to be drawn into the cylinder 22 upon movement of the piston assembly 30 towards the right in FIG. 2 to thereby lift the load 18, and movement of the pulleys 38 caused by movement of the piston assembly 30 in the opposite direction, that is towards the fixed pulleys 42, permits the cable 16 to be extended from the cylinder 22 to thereby lower the load 18. The distance the load 18 will be lifted or lowered by the cable 16 is determined by the length of the power stroke of the piston assembly 30 multiplied by the number of fixed and movable pulleys. This construction of a load balancing hoist assemblyis conventional and forms no part of the present invention other than in combination with the present improved fluid pressure control system, which will be now described in greater detail.

The front portion of chamber 28 of the cylindrical housing 22 between the front end cap 24 and the pressure piston 32 defines a fluid pressure chamber 46 sealed from the chamber 28 by means of a dynamic seal assembly 48 supported by the piston 32. The pressure chamber 46 is adapted to be filled with a fluid under pressure through an inlet orifice 50 extending through the end cap 24. The inlet orifice 50 is connected by means of a fitting 52 to a pilot fluid controlled fluid pressure regulator 54. The fluid pressure regulator 54 can be directly manually adjusted as illustrated in the embodiment in FIG. 2 or it may be pilot controlled as in the embodiment illustrated in FIG. 1.

Referring again to FIG. 1, the pilot pressure operated fluid pressure regulator 54 is connected by means of an inlet fitting 74 and a conduit 60 to a source of fluid under pressure 58. In the pilot controlled fluid pressure regulator system as schematically illustrated in FIG. 1 the fluid pressure regulator 54 is further provided with a pilot inlet 62 connected to a flexible pilot pressure conduit or hose 64 which is connected to the outlet side 66 of a remote pilot pressure regulator valve 68 the inlet of which is connected by a fluid conduit 70 to the source of fluid under pressure 58. The remote pilot regulator valve 68, as is common has an adjustable pressure relief for exhausting pressure as indicated by the arrow 72 so that upon adjustment of the pilot pressure regulator valve 68 to regulate exhaust, the pilot pressure delivered to the primary fluid pressure regulator 54 can be precisely adjusted.

As shown in detail in FIG. 3, the fluid pressure regulator valve 54 comprises a housing 106 having a lower constant pressure chamber 108 provided with an inlet 74 which is in fluid communication through a one way check valve 110 and conduit 60 with the source of fluid pressure 58 (FIG. 1). The constant pressure chamber 108 is selectively communicable with a regulator pressure chamber 112 which is connected by an outlet 52 and through a normally open safety valve 114 to the inlet 50 of the hoist cylinder pressure chamber 46 to supply that chamber with regulated predetermined fluid pressure at varying magnitudes.

A valve 116 is positioned intermediate the constant pressure chamber 108 and the regulator pressure chamber 112 and is normally retained in a closed position by fluid pressure and a spring 1 18. Valve 116 is provided with a relief valve 120 connected thereto by a stem 122 so the valves 1 16 and 120 act together to control the pressure within the regulator chamber 112 depending upon the opened or closed position of the valve members 116, 120. The relief valve 120 normally closes an exhaust passage 124 provided in an intermediate floating spacer member 126 for exhausting air from the chamber 112 to the atmosphere through an orifice 128. The spacer member 126 is supported by a lower and upper diaphragm 130 and 132 respectively and separates the regulator chamber 1 12 from a pilot pressure chamber 134. The pilot pressure chamber 134 is normally closed by a valve 138 connected to a pilot fluid pressure measuring capsule 140 the interior of which is fluid connected by an inlet passage 142 provided in an inlet fitting 62 at the top of the housing 106 to fluid line 64 which is connected to the outlet 66 of the pilot pressure regulator 68 (FIG. 1). The measuring capsule 140 is disposed within a control chamber 144 which is in open fluid communication with the regulator chamber 112 by means of a passage 146 provided in the side wall of the housing 106 so as to maintain substantially equal fluid pressure in both chambers 112 and 144.

Normally, the inlet valve 116 is maintained in closed position by fluid pressure in the constant pressure chamber 108 in addition to the force of the spring 118. Small movement of the measuring capsule 140 downwardly in the control chamber 144 as caused by admission of pilot fluid to the interior of the capsule 40 causes the pilot valve 138 to open to establish fluid communication between the control chamber 144 and the pilot chamber 134. The increased pressure in the pilot chamber 134 acts on the upper diaphragm with excess pressure being vented to the atmosphere through a vent 148. Pressure on the diaphragm 130 is excess of that in the chamber 112 produces downward movement of the spacer member 126 and the inlet valve 116 to admit fluid in metered amounts from the constant pressure chamber 108 to the regulator chamber 112 until pressure across the diaphragms 130 and 132 equalizes. Excess pressure acts on the lower diaphragm 132 to move the floating spacer member 126 away from the relief valve 120 to open the chamber 112 to the vent orifice 128. At the same time the spring 118 forces the inlet valve 116 into closed position to interrupt fluid communication between chambers 108 and 112. The increased pressure in the regulator chamber 112 is also transmitted through the passage 146 to the control chamber 144 to act on the measuring capsule to close the pilot valve 138 to again interrupt fluid communication between the control chamber 144 and the pilot chamber 134. In this way regulated pressure is delivered to the hoist chamber 46 in response to a signal from the pilot pressure regulator 68. The pressure of the fluid delivered to the inlet 62 will determine the pressure delivered to the hoist chamber 46 and at any setting of the pilot pressure regulator 62 an increase in pressure in the hoist chamber 46, as caused by movement of the piston 32 to the left (FIG. 2) to raise the load will override the pilot pressure regulator 68 and the pressure will be exhausted. A decrease in pressure in the hoist chamber 46 as caused by lifting the load will permit the pilot pressure to move the spacer member 126 downwardly to open the chamber 108 to the chamber 112 and thereby increase the pressure in the hoist chamber 46.

Referring again to FIG. 2 for a description of the fluid pressure regulator valve 54 provided with a manual control the regulator 54 has its inlet 74 directly connected through a oneway check valve 110 and a conduit 60 to the source of fluid under pressure 58. The pilot pressure inlet 62 in the embodiment in FIG. 2 is provided with a manual adjusting mechanism 78 providing a valve rod 80 fluidly connected to the inlet 62 and which is rotatable by means of a knurled hand wheel 82 to adjust venting of fluid from the capsule 140 (FIG. 3) and in this way provide means for adjusting the balancer to accommodate different loads. The valve rod 80 is provided with an axial passage 84 connecting the pilot section of the pressure regulator 54 with the atmosphere by means of suitable orifices 86. The axial passage 84 and the orifices 86 are comparable to the inlet 62 and pilot line 64 in FIG. 3 and are in fluid communication with the floatable pilot pressure measuring capsule 140 within the fluid pressure regulator valve 54, as shown in FIG. 3 and described above so that movement of the capsule as caused by changes in pressure in chamber 144 produces a regulating of pressure applied to the fluid pressure chamber 46 in the hoist housing cylinder 22 substantially as described with respect to the embodiment of FIGS. 1 and 3.

Thus, from the foregoing description it will be obvious that, upon attaching a load to the free end of the cable 16, direct manual or remotely controlled adjustment of the pilot pressure operated fluid pressure regulator 54 to increase the regulator output pressure supplied to the fluid pressure chamber 46 of the hoist cylinder 22 will balance a load by the provision of sufficient fluid pressure over the dynamic piston area of the pressure piston 32 to maintain the piston in a balanced" condition. The adjustment permits loads of different weights to be balanced and once the proper adjustment has been made the supplying of air of sufi'icient pressure to maintain the balancing will be automatically produced by the pressure regulator 54.

Upon pulling down on the load, the increased pressure caused by the load acting upon the piston to move it to the left (FIG. 2) will act upon the diaphragm 132 (FIG. 3) to move the member 126 upwardly. This causes valve 116 to close and 120 to open. Opening valve 120 permits bleeding of excess air pressure through the orifice 148 so that balance of the load is maintained as it is moved downwardly.

Manual movement of the load upwardly decreases pressure in chamber 112 permitting member 126 to be moved downwardly by pilot pressure to thereby close valve 120 and to cause valve 116 to be opened to open chamber 112 to pressure chamber 108 to thereby increase pressure in the cylinder chamber to maintain balancing of the load as it is moved upwardly.

Referring again to FIG. 2, the cylinder 22 is provided with an extension 90 secured to the rear end cap 26 and forming an extension of the chamber 98. Cap 26 is provided with an appropriate aperture 92 of such dimensions as to allow the pulleys 38 to move into the extension 90 during rearward movement of the piston assembly 30. Centrally, the extension 90 is provided with a check valve assembly 94 contained within an opening 96 forming a valve seat 98 for cooperation with a valve plate 100. The valve plate 100 is normally resiliently biased away from the valve seat 98 and against the head of a bolt 104 supporting the valve plate by means of a spring 102 supported within the opening 96. The bolt 104 forms a guide for the valve plate 100 and a nut 105 tightened against a perforated plate 106 maintains the bolt 104 and valve plate 100 in position. The arrangement of the check valve assembly 94 in the extension 90 is adapted to control the raising or wind-up speed of the cable 16 relative to the lifting of the load 18 and to dampen the rearward movement of the piston assembly 30 upon an increase of pressure in the fluid pressure chamber 46. Thus, as seen in FIG. 2, when the piston assembly 30 is moved rearwardly, that is towards the right in FIG. 2, by means of an increase in pressure in the pressure chamber 46, the valve plate 100 will be forced from its normally open position toward a closed position against the valve seat 98 to thereby trap a certain amount of atmospheric pressure within the rear end of the cylinder chamber 28. The rearward movement of the piston assembly 30 and the raising or winding-up of the cable 16 is thus dampened by the check valve assembly 94 to prevent a sudden rearward movement of the piston assembly 30 upon an increase of pressure in the pressure chamber 46 when, for instance, the load is removed from the cable 16. Upon unwinding of the .cable 16 from the cylinder 22 for lowering of the load 18 by means of a decreased pressure in the fluid pressure chamber 46, causing movement of the piston assembly 30 towards the left in FIG. 2, a suction force will be created in the rear portion of the cylinder chamber 28 and in the extension 90, and the spring 102 then will move the plate to an open position to thereby admit atmospheric pressure into the extension 90 behind the piston portion 34 to facilitate movement of the piston assembly 30.

Thus, it will be seen from the foregoing description in connection with the drawings that an improved fluid flow control system for a fluid pressure operated hoist mechanism has been provided including a fluid pressure regulator valve operable to compensate for various fluid pressure conditions caused by the weight balanced by the hoist and by the weight being lowered and raised so as to maintain the weight of the load in balance at all times.

Additionally, atmospheric pressure responsive valve means has been provided which functions to dampen the movement of the hoist piston upon a sudden decrease in load such as caused by removing the weight supported by the hoist to produce a sudden increase in fluid pressure against the piston in the hoist chamber.

Although only two preferred embodiments have been shown and described herein, it is obvious that various modifications may be made without departing from the spirit and essential characteristics of the present invention as defined by the scope of the appended claims.

I claim: v 1. In combination, a pneumatically operated hoist having a hoist housing, a piston assembly movable in said housing, a first set of pulleys fixed within said housing, a second set of pulleys within the housing supported by said assembly for movement therewith towards and away from said first set of pulleys, a cable wound around said first and said second set of pulleys having one end anchored in said housing, the other end of said cable extending out from said housing for attachment to and support of a load therein, a fluid pressure chamber defined between the front end of said piston assembly and one end of said housing, means supplying said fluid pressure chamber with fluid under pressure to move said piston assembly and said second set of pulleys towards or away from said first set of pulleys, said means comprising a fluid pressure regulator valve, said valve having an inlet connected with a source of fluid pressure and an outlet connected with said fluid pressure chamber of said hoist, a normally closed first valve intermediate said inlet and said outlet, a normally closed relief valve connected with said first valve and operable when opened to relieve pressure from said outlet, a floating spacer member movable between a first position opening said first valve and a second position opening said relief valve, said spacer member being subjected to outlet pressure on one side and pilot pressure responsive means regulating pressure to the opposite side of said spacer member, said pilot pressure responsive means comprising a control chamber connected with said outlet, a capsule disposed within said control chamber with its interior connected to a pressure line extending between said capsule and said source and having a pilot regulator valve therein to provide a source of pilot pressure in said chamber, and a second valve connected with said capsule and operable to regulate pressure from said control chamber to side of said spacer member opposite said outlet pressure whereby an increase in pilot pressure or a decrease in outlet pressure opens said second valve to deliver pressure fluid to said opposite side of spacer member to cause said first valve to open and an increase in outlet pressure moves said spacer member to a position opening said relief valve and said hoist is converted to a balancer.

2. The device as defined in claim 1 and including means carried by said housing to dampen the movement of said piston assembly rearwardly upon a sudden increase in pressure within said pressure chamber.

3. The load balancing device as defined in claim 2, in which said last mentioned means comprises said housing being provided with a rearwardly extending end portion, said end portion having an aperture to provide for the movement of said pulleys supported by said piston assembly into said end portion, said end portion comprising a compensating chamber provided with a one-way check valve normally biased in an open position to communicate said end portion with atmosphere but operable upon an increase to close to thereby dampen the movement of said piston assembly in a direction away from said fixed set of pulleys.

4. The device as defined in claim 3, said valve comprising an opening in said housing, a stem extending through said opening having an enlarged head portion disposed inwardly of said housing, said enlarged head portion supporting a valve plate on said stern having a larger diameter than said opening, a plate disposed at the other end of said stem outwardly of said housing and overlying said opening, and fastening means securing said valve plate in position. v

5. In the device as defined in claim 4, in which the inner end of said opening defines a valve seat in said valve plate being adapted to cooperate with said valve seat to close said opening.

6. In the device as defined in claim 5, spring means disposed in said opening around said stem to normally bias said valve plate away from said valve seat.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 411 Dated June 13, 1972 Inventor(s) Lorne J; McKendrick It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 53, after "cylinder to", insert -thereby cause the cable to be drawn into the hoist cylinder to-.

Signed .and sealed this 26th day of September 1972.

(SEAL) Attest:

EDWARD M.FLET( IHER,JR. ROBERT GOTTSCHALK Attestlng Officer Commissioner of Patents FORM P0-1050 (10- v USCOMM-DC 60376-P69 a [1.5. GOVERNMENT PRINTING OFFICE I 1959 9-355-334 

1. In combination, a pneumatically operated hoist having a hoist housing, a piston assembly movable in said housing, a first set of pulleys fixed within said housing, a second set of pulleys within the housing supported by said assembly for movement therewith towards and away from said first set of pulleys, a cable wound around said first and said second set of pulleys having one end anchored in said housing, the other end of said cable extending out from said housing for attachment to and support of a load therein, a fluid pressure chamber defined between the front end of said piston assembly and one end of said housing, means supplying said fluid pressure chamber with fluid under pressure to move said piston assembly and said second set of pulleys towards or away from said first set of pulleys, said means comprising a fluid pressure regulator valve, said valve having an inlet connected with a source of fluid pressure and an outlet connected with said fluid pressure chamber of said hoist, a normally closed first valve intermediate said inlet and said outlet, a normally closed relief valve connected with said first valve and operable when opened to relieve pressure from said outlet, a floating spacer member movable between a first position opening said first valve and a second position opening said relief valve, said spacer member being subjected to outLet pressure on one side and pilot pressure responsive means regulating pressure to the opposite side of said spacer member, said pilot pressure responsive means comprising a control chamber connected with said outlet, a capsule disposed within said control chamber with its interior connected to a pressure line extending between said capsule and said source and having a pilot regulator valve therein to provide a source of pilot pressure in said chamber, and a second valve connected with said capsule and operable to regulate pressure from said control chamber to side of said spacer member opposite said outlet pressure whereby an increase in pilot pressure or a decrease in outlet pressure opens said second valve to deliver pressure fluid to said opposite side of spacer member to cause said first valve to open and an increase in outlet pressure moves said spacer member to a position opening said relief valve and said hoist is converted to a balancer.
 2. The device as defined in claim 1 and including means carried by said housing to dampen the movement of said piston assembly rearwardly upon a sudden increase in pressure within said pressure chamber.
 3. The load balancing device as defined in claim 2, in which said last mentioned means comprises said housing being provided with a rearwardly extending end portion, said end portion having an aperture to provide for the movement of said pulleys supported by said piston assembly into said end portion, said end portion comprising a compensating chamber provided with a one-way check valve normally biased in an open position to communicate said end portion with atmosphere but operable upon an increase to close to thereby dampen the movement of said piston assembly in a direction away from said fixed set of pulleys.
 4. The device as defined in claim 3, said valve comprising an opening in said housing, a stem extending through said opening having an enlarged head portion disposed inwardly of said housing, said enlarged head portion supporting a valve plate on said stem having a larger diameter than said opening, a plate disposed at the other end of said stem outwardly of said housing and overlying said opening, and fastening means securing said valve plate in position.
 5. In the device as defined in claim 4, in which the inner end of said opening defines a valve seat in said valve plate being adapted to cooperate with said valve seat to close said opening.
 6. In the device as defined in claim 5, spring means disposed in said opening around said stem to normally bias said valve plate away from said valve seat. 